1. Field of the Invention
This invention relates to a process for reducing tensile welding stresses in nozzles in a nuclear reactor shell, preferably in a nuclear reactor head, said nuclear reactor shell having been provided with nozzles extending through openings in said shell, said nozzles having been welded to said nuclear reactor shell through a weld lying on an inner surface of said nuclear reactor shell and the outer surface of said nozzle immediately adjacent thereto, said welding resulting in the generation of high tensile welding stresses in the wall of said nozzle adjacent to said weld, which comprises applying axial compressive loads, or pressures, on the free ends of said nozzles sufficient to substantially reduce said welding stresses.
2. Description of the Prior Art
Nuclear reactors are normally provided in the shell thereof, particularly in the head thereof, with nozzles generally extending substantially vertically through openings therein and which are circumferentially welded to said shell through a weld lying on the inner surface of said shell and on the outer surface of said nozzle immediately adjacent thereto. These nozzles, particularly when they extend through the head of a nuclear reactor, generally are made of alloys of nickel or stainless steel, such as an Inconel alloy, such as Inconel 600, provide for passage therethrough of control rod drive shafts attached to an exterior control rod drive mechanism. The tensile stresses generated from the weld holding the nozzle to the nuclear reactor head tend to facilitate stress corrosion crack formation in the inner surface of the nozzle in the vicinity of said weld. These cracks, in time, can extend upwardly into the body of the nozzle or into a portion of the weld, or both, into communication with the outer surface of the nozzle above the weld, into the space between the outer surface of the nozzle and the adjacent surface of the head opening, and then to the exterior of the reactor. Irradiated water can thus pass upwardly into the atmosphere, creating a serious pollution problem. If the cracks are circumferential, the nozzle can be detached from the reactor head by the pressure in the reactor, also with damaging results.
As seen in U.S. Pat. No. 5,196,160, filed Mar. 23, 1992, one way to obviate such problems is to form a coaxial circumferential groove extending upwardly into at least a portion of the nozzle that extends into the reactor and then filling such circumferential groove with a weld material.
I have now found that such problems in the nozzle arising from the tensile stresses generated in the wall of said nozzle adjacent to the weld holding said nozzle to said nuclear reactor shell, particularly a nuclear reactor shell, can be substantially reduced by applying axial compressive loads to the free ends of said nozzles.